In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art.
Structures having nanometer-scale dimensions can possess unique and useful properties, such as the quantum behaviors observed in some nanostructures.
One such material is silicon nanowires. The nanowires are characterizable as one-dimensional, small-scale, large surface area wire-like materials that exhibit electron transport, photoluminescent and/or quantum effects. Such materials are of interest for certain medical and electronics applications.
A number of techniques have been described for making such structures. These techniques include lithography, plasma etching, plasma deposition, reactive ion etching, chemical vapor deposition, laser ablation, sputtering, thermal evaporation decomposition, electron-beam evaporation, supercritical vapor-liquid solid synthesis, electrochemical dissolution, and metal-induced local oxidation and dissolution.
U.S. Pat. No. 5,348,618 discloses a chemical dissolution procedure reportedly for forming silicon nanowires. The procedure comprises a first anodizing step, followed by a second chemical dissolution step to increase pore size. No visual evidence (e.g., imaging) is included in the patent to demonstrate that well-aligned silicon nanowires along the (001) direction were actually achieved.
U.S. Pat. No. 5,458,735 describes a process for forming a “microporous silicon layer” having luminescent properties. The process described therein involves illuminating the anode side of a silicon wafer during at least part of the time in which the wafer is placed in an acidic solution. The microporous layer comprises a n+ doping region in addition to a p+ doping region, thereby forming a p-n junction. The patent does not discuss formation of silicon nanowires.
U.S. Pat. No. 5,552,328 describes the formation of porous silicon light emitting diode arrays. The only discussion of the morphology of the porous silicon describes “a column-like Si structure.” The columns are said to have a diameter on the order of 50-100 nanometers. The porous silicon is produced by a electrochemical dissolution process. The reported etching current is 10 mA/cm2, and the etching solution was HF:H2O=1:3. The process is described as taking place with illumination. Moreover, most embodiments described therein involve complicated silicon structures such as p−n junctions or “poly-Si.”
A thesis entitled “Investigation of Pristine and Oxidized Porous Silicon” by Andrea Edit of the University of Oulu, dated 2005 discusses the synthesis and properties of porous silicon. It is noted in the thesis that porous silicon can comprise crystalline silicon nanowires. However, the thesis does not describe the morphology or nature of silicon nanowires, or the mechanism(s) of their formation, in any detail. Moreover, the thesis does not describe free-standing nanowires or techniques for obtaining them from a substrate.
Despite the numerous techniques utilized, a need still exists for a simple and cost-effective fabrication method to form well-aligned, high-aspect ratio silicon nanowires in large quantities with tailorable geometries.